Gas turbine engines typically include high and low pressure compressors, a combustor, and at least one turbine. The compressors compress air, which is mixed with fuel and channeled to the combustor. The mixture is then ignited and generates hot combustion gases. The combustion gases are channeled to the turbine which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to propel an aircraft in flight or to power a load, such as an electrical generator. In the instance of an aircraft engine, the useful work may take the form of a jet exhaust stream. Alternatively, the turbine may power a propeller or a fan.
Though highly unlikely, it is postulated that a fan blade in the turbine engine may become damaged and tear loose from the rotor during operation. It is further postulated that this unlikely event would cause the damaged fan blade to pierce the surrounding engine housing, which may result in cracks along the exterior surface of the engine housing. To alleviate concerns associated with a postulated fan blade separation event, at least some known engines are assembled with an inlet housing shell that increases the radial and axial stiffness of the engine and that reduces stresses near the engine housing penetration.
Known fan inlet housing assemblies include a double-walled structure. The primary and secondary walls are typically separated by a distance of one or more inches, depending on the size of the engine, in order to provide structural redundancy. Some existing fan inlet housings include either an aluminum alloy or a graphite epoxy composite material for the primary wall, a graphite epoxy composite material for the secondary wall, and a low density honeycomb or foam core material to effect load transfer between the walls. In these housings, the core is typically arranged between the walls in a “sandwich” configuration.
FIGS. 1a and 1b depict an example double-walled assembly 100 (FIG. 1a is provided in axial cross-section with respect to the engine housing, and FIG. 1b is provided in radial cross-section). The primary wall 110 extends axially to a length L1. The length L1 is dependent on the size of the engine, and can readily be selected by a person of ordinary skill in the art. In some implementations, the length L1 can range from approximately 8 inches to approximately 36 inches, although smaller and larger lengths are possible. The primary wall 110 includes two flanges 111a,111b located at each axial end thereof. The flanges 111a,111b are oriented generally perpendicularly with respect to the axial length of the wall 110. Adjoined to the primary wall 110 is the secondary wall 120. The secondary wall 120 extends axially along the length of the primary wall 110 at a length L2, which is less than L1, and can range from approximately 4 inches to approximately 18 inches, although smaller and larger lengths are possible. The secondary wall 120 can be a generally trapezoidal shape, having flanges 121a,121b oriented parallel to primary wall 110 for adhesively adjoining the secondary wall 120 thereto. The secondary wall has a width W1, which can typically range from approximately 1 to 3 or more inches, depending on the strength characteristics desired. Positioned (“sandwiched”) between the primary wall 110 and the secondary wall 120 is a honeycomb or foam material core layer 130.
To fabricate the assembly 100, in the past, the graphite epoxy composite wall structures 110,120 were molded under high pressure (for example, approximately 50-100 psi was a typical molding pressure) to achieve the desired properties (strength, load transfer, etc.). In practice, it was observed that high molding pressure caused the low density core 130 to collapse, so the individual walls 110,120 were molded separately, and then assembled to the core 130 using an adhesive. The process was labor-intensive and had an undesirably high fabrication cost.
With reference to FIGS. 2a and 2b, the current state of the art typically employs a three-step process (illustrated as process flow chart 200a in FIG. 2a and as schematic flow diagram 200b in FIG. 2b). First, at step 201a,201b, the primary wall 110 is fabricated as a separate detail by high pressure molding, using mold 140, if a composite material is used or by machining if a metal material is used. Second, at step 202a,202b, an adhesive 125 is applied to bond the low-density core 130 to the primary wall 110. Third, at step 203a,203b, in-situ fabrication techniques are employed to shape the secondary wall 120 directly to the core 130 and primary wall 110 using low pressure molding (arrows 122). For example, approximately 10-15 psi is a typical molding pressure, to avoid collapse of the low density core. This approach results in less than optimal properties in the secondary wall (as compared to past techniques where both were high-pressure molded, then adhered to the core), but somewhat reduces fabrication complexity and therefore cost. As such, there is currently a trade-off between material properties and fabrication cost.
In an alternative process, a high-density core material could be used, thereby allowing for high-pressure molding to achieve the desired wall characteristics. However, in many applications, high-density core materials add an unacceptable amount of weight to the housing, and are therefore impractical.
Therefore, what is needed in the art is an improved method for manufacturing a composite fan inlet housing that achieves optimal material properties, including high strength and low weight, while maintaining a low cost of fabrication.